Integrated Optical Circuits (IOC) (also known as Photonic Integrated Circuits (PIC); the two terms will be used interchangeably herein) have been under development in many laboratories and companies for over three decades. In an analogy to electronic integrated circuits, developers of IOC envision the possibility of combining several or many optical processing functions on a single miniature platform, such as a semiconductor chip, fabricated using processes similar to those used for electronic chip production. Planar Optical Chips (POC) incorporate functional optical components such as linear or curved waveguides to conduct light from one location to another, filters fabricated from specially shaped waveguides that control the spectral characteristics of the light, and lenses and mirrors embedded in waveguides to alter the shape of the light. The POC are interfaced to other optical components and devices via optical fibers.
The waveguide components in IOC generally comprise several layers of materials. In an exemplary two-dimensional POC waveguides, a core layer of a material is covered on at least one side or, alternatively, sandwiched between two layers of clad material. The core material has a higher refractive index than the clad material. Similarly, in three-dimensional linear or curved waveguides, such as the common optical fiber, a core material is fully surrounded by a clad material.
Optical fibers are often used to transmit light signals in optical circuits and are examples of low-index-contrast waveguides. It is known, however, that low-index-contrast material systems are not optimum for IOC. High-index-contrast material systems, such as a core layer of silicon having a refractive index of approximately 3.5 clad with silica having a refractive index of approximately 1.5, offer stronger light confinement in smaller dimensions. Silica used as an insulating layer on silicon is also referred to as “oxide” or “insulator.” The stronger light confinement enables miniaturization of functional optical components to sizes that are comparable to the wavelength of the confined light, and thereby enables dense integration of these optical devices on waveguide chips.
The large mismatch between the common optical fiber dimension and the high-index-contrast waveguide dimension, and their respective mode sizes, complicates coupling of light from one to the other. A number of techniques have been utilized for optical coupling between these thin waveguides and conventional optical fibers, including prism couplers, grating couplers, tapered fibers and micro-lens mode transformers. None of these coupling techniques offer the combination of high coupling efficiency, wavelength independence, ease of access to remote portions of chips, reliability, manufacturability, ruggedness, and robustness demanded for use in low-cost high-volume telecommunication component production. These techniques often require time-consuming and complex procedures to facilitate coupling and often cannot readily provide access to remote portions of devices.
In conventional semiconductor-chip optical waveguide technology, silicon waveguide chips are cleaved and polished and optical fibers are butt coupled to the polished surface for light (often laser light) injection. In addition to time consumption, this process suffers from the difficulty inherent in cleaving the chip and cleaning the debris created by the polishing process from the chip without damaging the chip itself.
Current silicon based optical circuits utilize processes compatible with complimentary metal-oxide semiconductor (CMOS) technology when fabricating circuits. Silicon strip waveguides produced using CMOS compatible processes often have submicron cross-sections. The small cross-section, while a positive for fabricating small devices, makes conventional coupling technology very difficult as the cleaving and polishing that is normally required is hampered by the sub-micron size of the devices.
Devices fabricated from CMOS compatible processes have highly confined optical modes, due to the large difference between the refractive indices of the two materials, allowing for densely integrated optical waveguides and small radius of curvature waveguide bends. However, this carries with it an inherent problem. The small size of the silicon strip waveguides makes coupling them with optical fibers challenging due to the difference in mode field diameters (MFD's) between the optical fibers and the waveguides themselves. The large difference in MFD's can lead to coupling losses of more than 20 dB. There exists a need for combining the use of CMOS silicon circuits with a more efficient, accessible, and compatible technology for coupling a laser light source to devices on the entire chip surface.